Curable coating compositions such as thermoset coatings are widely used in the coatings art. They are often used for topcoats in the automotive and industrial coatings industry. Color-plus-clear composite coatings are particularly useful as topcoats where exceptional gloss, depth of color, distinctness of image, or special metallic effects are desired. The automotive industry has made extensive use of these coatings for automotive body panels. Color-plus-clear composite coatings, however, require an extremely high degree of clarity in the clearcoat to achieve the desired visual effect. High-gloss coatings also require a low degree of visual aberations at the surface of the coating in order to achieve the desired visual effect such as high distinctness of image (DOI).
As such, these coatings are especially susceptible to a phenomenon known as environmental etch. Environmental etch manifests itself as spots or marks on or in the finish of the coating that often cannot be rubbed out.
It is often difficult to predict the degree of resistance to environmental etch that a high gloss or color-plus-clear composite coating will exhibit. Many coating compositions known for their durability and/or weatherability when used in exterior paints, such as high-solids enamels, do not provide the desired level of resistance to environmental etch when used in high gloss coatings such as the clearcoat of a color-plus-clear composite coating.
Many compositions have been proposed for use as the clearcoat of a color-plus-clear composite coating, such as polyurethanes, acid-epoxy systems and the like. However, many prior art systems suffer from disadvantages such as coatability problems, compatibility problems with the pigmented basecoat, solubility problems. Moreover, very few one-pack coating compositions have been found that provide satisfactory resistance to environmental etch, especially in the demanding environment of automotive coatings.
Many curable coating compositions utilize a hydroxy-functional polymer resin such as a hydroxy-functional acrylic and a curing agent such as an aminoplast. These coating compositions suffer from environmental etch in certain topcoat applications. In spite of this, it is often desirable to use coatings based on hydroxy-functional or other active hydrogen-functional resins, as there exists a great deal of experience with these coatings, and many multilayer coating systems have incorporated this chemistry into one or more of the layers. It is also desirable to use such coatings for various applications such as basecoat and primer to provide durable coatings. It is especially desirable to utilize such coating compositions in high gloss topcoats, such as the clearcoat of a color-plus-clear composite coating, while also providing resistance to environmental etch.
U.S. Pat. Nos. 4,814,382, 5,114,015, and 5,158,808 describe the use of certain N-alkyl carbamate compounds as reactive diluents in coating compositions having OH-functional curable polymer resins. These compounds, however, may require excessively-high catalyst or temperature levels in order to fully react into the crosslink matrix during cure of the film.
WO 87/00851 describes the use of certain reactive carbamate derivatives in an effort to minimize the emission of volatile organic compounds (VOC). U.S. Pat. No. 5,744,550 describes the use of primary carbamate additives. However, further reductions in VOC are desirable without loss of desirable performance properties such as etch resistance and the like.